A secondary isotopic reference material of chlorine as NaCl collected from a sea water, located at 4degrees18'N, 161degrees08'E and collected on October 22, 1994, was prepared using ion exchange technique. The delta(37)Cl of an NaCl product named ISL 354 was measured to be - 0.39 +/- 0.05parts per thousand (2sigma) related to NIST SRM 975, an isotopic standard reference material with an kabsolute Cl-37/Cl-35 ratio of 0.31977. No contamination or isotopic fractionation was observed due to a complete recovery of chlorine and the avoidance of chemical reagents during preparation of ISL 354 from selected seawater. The isotopic composition of chlorine in ISL 354 does not change on long-term storage. So ISL 354 can be used as a secondary isotopic reference material of chlorine. (C) 2002 Elsevier Science B.V. All rights reserved. A secondary isotopic reference material of chlorine as NaCl collected from a sea water, located at 4degrees18'N, 161degrees08'E and collected on October 22, 1994, was prepared using ion exchange technique. The delta(37)Cl of an NaCl product named ISL 354 was measured to be - 0.39 +/- 0.05parts per thousand (2sigma) related to NIST SRM 975, an isotopic standard reference material with an kabsolute Cl-37/Cl-35 ratio of 0.31977. No contamination or isotopic fractionation was observed due to a complete recovery of chlorine and the avoidance of chemical reagents during preparation of ISL 354 from selected seawater. The isotopic composition of chlorine in ISL 354 does not change on long-term storage. So ISL 354 can be used as a secondary isotopic reference material of chlorine. (C) 2002 Elsevier Science B.V. All rights reserved.